Add KtLintAssertThat for asserting unit tests in a fluent style

CHANGELOG.md

@@ -4,11 +4,20 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## Unreleased
 
+### API Changes & RuleSet providers
+
+If you are not an API user nor a RuleSet provider, then you can safely skip this section. Otherwise, please read below carefully and upgrade your usage of ktlint. In this and coming releases, we are changing and adapting important parts of our API in order to increase maintainability and flexibility for future changes. Please avoid skipping a releases as that will make it harder to migrate.
+
+#### Testing KtLint rules
+
+An AssertJ style API for testing KtLint rules ([#1444](https://github.com/pinterest/ktlint/issues/1444)) has been added. Usage of this API is encouraged in favor of using the old RuleExtension API. For more information, see [KtLintAssertThat API]( https://github.com/pinterest/ktlint/blob/master/ktlint-test/README.MD)
+
 ### Added
 
 ### Fixed
 
 ### Changed
+
 * Set Kotlin development version to `1.6.21` and Kotlin version to `1.6.21`.
 
 ### Removed

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/EnumEntryNameCaseRule.kt

@@ -8,7 +8,6 @@ import org.jetbrains.kotlin.psi.KtEnumEntry
 public class EnumEntryNameCaseRule : Rule("enum-entry-name-case") {
 
     internal companion object {
-        const val ERROR_MESSAGE = "Enum entry name should be uppercase underscore-separated names like \"ENUM_ENTRY\" or upper camel-case like \"EnumEntry\""
         val regex = Regex("[A-Z]([A-Za-z\\d]*|[A-Z_\\d]*)")
     }
 
@@ -26,7 +25,7 @@ public class EnumEntryNameCaseRule : Rule("enum-entry-name-case") {
         if (!name.matches(regex)) {
             emit(
                 node.startOffset,
-                ERROR_MESSAGE,
+                "Enum entry name should be uppercase underscore-separated names like \"ENUM_ENTRY\" or upper camel-case like \"EnumEntry\"",
                 false
             )
         }

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/PackageNameRule.kt

@@ -5,10 +5,6 @@ import com.pinterest.ktlint.core.ast.ElementType.PACKAGE_DIRECTIVE
 import org.jetbrains.kotlin.com.intellij.lang.ASTNode
 import org.jetbrains.kotlin.psi.KtPackageDirective
 
-/**
- * @see [Kotlin Style Guide](https://kotlinlang.org/docs/reference/coding-conventions.html#naming-rules)
- * @see [Android Style Guide](https://android.github.io/kotlin-guides/style.html#package-names)
- */
 class PackageNameRule : Rule("package-name") {
 
     override fun visit(

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/trailingcomma/TrailingCommaRule.kt

@@ -328,6 +328,8 @@ public class TrailingCommaRule :
 
         private val BOOLEAN_VALUES_SET = setOf("true", "false")
 
+        // TODO: Rename property to trailingCommaOnDeclarationSite. The word 'allow' is misleading as the comma is
+        //  enforced when the property is enabled and prohibited when disabled.
         public val allowTrailingCommaProperty: UsesEditorConfigProperties.EditorConfigProperty<Boolean> =
             UsesEditorConfigProperties.EditorConfigProperty(
                 type = PropertyType.LowerCasingPropertyType(
@@ -345,6 +347,8 @@ public class TrailingCommaRule :
                 "should be enforced on the calling side," +
                 "e.g. argument-list, when-entries, lambda-arguments, indices, etc."
 
+        // TODO: Rename property to trailingCommaOnCallSite. The word 'allow' is misleading as the comma is
+        //        //  enforced when the property is enabled and prohibited when disabled.
         public val allowTrailingCommaOnCallSiteProperty: UsesEditorConfigProperties.EditorConfigProperty<Boolean> =
             UsesEditorConfigProperties.EditorConfigProperty(
                 type = PropertyType.LowerCasingPropertyType(